The present invention pertains to the electrical tuning art and, more particularly, to an improved cavity resonator.
Cavity resonators are well known, especially in the radio communication art. There, cavity resonators are used to provide selectivity at very high frequencies. A type of cavity resonator, known as a helical resonator, is generally comprised of a helically wound coil positioned within a resonant cavity. By appropriate adjustment of a provided tuning screw, the effective capacitance between the coil and the cavity is adjusted such that a series resonant LC circuit is formed. Several resonators are commonly coupled to provide the overall selectivity requirements of, for example, a radio receiver.
A principle problem with prior art helical resonators is temperature stability. For applications wherein a wide ambient temperature range is experienced, such as in mobile communication equipment, substantial drifts in the center frequency of the helical resonators have been experienced. In the prior art, the temperature drift of cavity resonators has been compensated for in at least one of three ways. Firstly, precision components may be used which exhibit very tight temperature characteristics. This approach results in a resonator which is expensive to manufacture. A second approach has been to broad tune the resonators such that the substantial thermal drifts can be tolerated. This approach is undesirable in that it sacrifices selectivity. A third approach sacrifices the tuning sensitivity for enhanced temperature sensitivity.